The present invention relates to a process for fabrication of rack shaft for use in steering gears for vehicles and a fabrication machine therefor.
There have conventionally been provided rack and pinion steering gears as the steering units for vehicles. The rack and pinion steering gear is arranged such that a pinion is meshed with rack teeth cut on a rack shaft so as to axially move the rack shaft as rotated by the operation of a steering wheel, thereby allowing the steering of a vehicle.
As shown in FIGS. 12 and 13, the rack shaft for use in such a rack and pinion steering gear is formed of a steel pipe P for weight reduction. The steel pipe P is cut with rack teeth 90 at its axial intermediate portion, which rack teeth are meshed with the pinion. The rack teeth 90 are formed by a process comprising the steps of working a flat portion 92 by plastically deforming a part of the steel pipe P with an arcuate portion 91 thereof kept intact, and subjecting the flat portion 92 to a gear cutting work. The rack teeth 90 portion is hardened to a predetermined hardness by means of induction hardening.
As a process for forming the above flat portion 92 of the rack shaft, press forming is generally adopted wherein as retained by a die 93 including an upper die 93a and a lower die 93b, the steel pipe P is pressed by a forming punch 94 having a flat bottom surface, as shown in FIGS. 14 and 15. Unfortunately, however, this process produces sudden plastic deformation of the steel pipe P and therefore, dent-like deformations tend to occur at places axially and peripherally central of the flat portion 92, as shown by two-dot chain lines in the figures, or cracks tend to occur at places subject to a great quantity of deformation, such as inside corner portions Pa where an inner side of the flat portion 92 joins with an inner side of the arcuate portion 91.
To overcome this drawback, a work process has been proposed such that a bar-like jig 95 slantly positioned relative to the axis of the steel pipe P is pressed against the steel pipe P and, in this state, the jig is moved axially of the steel pipe P for forming the flat portion 92 (see Japanese Examined Patent Publication No. 5(1993)-88662).
However, the process utilizing the bar-like jig 95 has a problem that the jig 95 and the steel pipe P are in sliding contact under a great pressure and therefore, frictional resistance during the sliding movement may sometimes cause the both to seize up despite lubrication. Accordingly, there exists a need for preliminarily subjecting the steel pipe P to a lubrication process, such as bonderlube, for prevention of the occurrence of the aforesaid seizing. This results in an increased production cost.